Direct current power systems for coal mines are generally fed from several d.c. power substations at suitable spaced intervals (typically one mile) throughout the coal mine. The output power from the substations is fed through d.c. circuit breakers which will trip, and thus disconnect the d.c. power supply, when excessive current is being drawn.
Such d.c. circuit breakers must be calibrated such that the trip level or setting is high enough to permit normal load currents but sensitive enough to provide tripping of the breaker for low resistance, high current faults. Moreover, periodic calibration of the overcurrent trip setting of such breakers is necessary to ensure continued protection and safety for the mine system. In addition, on site adjustment and recalibration often becomes necessary as the mine workings advance and the requirement of the power system change.
The calibration techniques which are presently used for on site calibration of d.c. circuit breakers suffer a number of important disadvantages. In general, these techniques are cumbersome and because of the large amount of power required, potentially dangerous. For example, one known method uses large power resistor which have ratings of about 300 volts and 3600 amperes and which must be hauled to the circuit breaker location to perform the necessary testing. Another approach relies on the use of two locomotives to draw a large current from the breaker, thereby causing the breaker to trip. It is evident that neither of these appraoaches provides a convenient and accurate calibration test for circuit breakers.
More generally, other techniques have been used for in service testing of electromagnetic devices such as solenoids, relays and the like. An example of such a technique is disclosed in U.S. Pat. No. 3,976,933 (Banfi). This patent relates to a testing wherein a current is caused to pass through a normally de-energized coil to effect energization thereof or, conversely, for a normally energized coil, to effect de-energization thereof. A power source is provided which is separate from the power source being controlled by the device under test and, considering the example of a normally de-energized coil, currrent is passed for a short period of time so that various portions of the test system can be checked without activating the whole system. The Banfi system is a low power system and a simple low power transistor device is used to control the test current. Other patents of possible interest include U.S. Pat. Nos. 2,614,150 (Bucker); 3,604,976 (Zajic); 3,424,958 (Groat) and 3,852,660 (Maier), although this listing is obviously not represented to be exhaustive.